The present invention relates to a laser beam projecting device comprising structure for shutting off a return beam of a laser beam emitted from a laser light source.
It is known that output of a laser light source for emitting a laser beam is decreased when there is a return beam, and a surveying system utilizing the laser beam has an arrangement to shut off the return beam.
FIG. 7 shows general features of an optical system of a conventional type laser beam projecting device, which has an operation to shut off the return beam.
In FIG. 7, reference numeral 1 denotes a semiconductor laser, and reference numeral 2 denotes a condenser lens. Reference numeral 3 denotes an anamorphic prism, which comprises two wedge-like prisms 4a and 4b having different collection angles in two directions of cross-section of luminous flux. After passing through the anamorphic prism 3, a laser beam 5 is projected on a projection optical axis 6. On the projection optical axis 6, there are arranged a polarizing plate 7 for transmittig P-polarized light and a ¼λ plate (birefringent optical component) 8. Reference numeral 9 denotes an optical component for reflecting the laser beam 5. The optical component is a reflecting mirror, for instance.
The laser beam 5 emitted from the semiconductor laser 1 is, for instance, a linearly P-polarized light, and cross-section of luminous flux of the laser beam 5 is in elliptical shape. After the laser beam 5 is turned to parallel luminous flux by the condenser lens 2, the cross-section of the luminous flux of the laser beam 5 is expanded in a short axis direction by the two wedge-like prisms 4a and 4b of the anamorphic prism 3, and the laser beam 5 is shaped so that the cross-section of the luminous flux has circular shape.
After passing through the anamorphic prism 3, the laser beam 5 with the luminous flux turned to circular shape passes through the polarizing plate 7. The laser beam 5 is then converted to a circularly polarized light by the ¼λ plate 8 and is projected.
Being reflected by the optical component 9, the circularly polarized reflected laser beam 5′ is converted to a linearly polarized light as the reflected laser beam 5′ passes through the ¼λ plate 8. Further, the linearly polarized reflected laser beam 5′ thus converted has a direction of polarization different by 90° with respect to the projected laser beam 5, and the reflected laser beam 5′ becomes S-polarized light. The polarizing plate 7 is arranged so as to allow the P-polarized light to pass. Accordingly, the S-polarized reflected laser beam 5′ is shut off by the polarizing plate 7 and does not reach the semiconductor laser 1.
In the conventional type laser beam projecting device as described above, the returning of the reflected laser beam 5′ is shut off by a combination of the polarizing plate 7 and the ¼λ plate 8 arranged on the projection optical axis 6, and the polarizing plate 7 is required to be arranged on the projection optical axis 6. This results in complicated structure of the optical system. Also, as the polarizing plate 7 is provided, reflection occurs on both surfaces of the polarizing plate 7, and this causes loss of the laser beam.